i have built and been around many mud trucks and 3:08s with 35s and a 350 is not a good combo. especially with a th700. currently a have an 87 4x4 with a 454tbi swap, 35s and 4:10s. it revs at 1800 at 60mph. pulls it very nice.
and swirl ports do make good low end torque. its the HP that suffers. it is very likely you just need a gear swap. at least 4:10s will fix your problem.
and the last thing i would ever do to a mud truck is ditch the EFI. they run great, especially on angles. i would never again in my life put a complete piece of junk carb like an edelbrock on one.